This invention relates to telephone systems and particularly to subscriber set ringing circuits.
A conventional telephone subset includes a bell or ringer which is used for alerting the subscriber and is operated by applying a high voltage low frequency ringing signal on the subscriber line. Prior telephone switching networks have provided a metallic path for each connection whereby a high voltage ringing signal could be transmitted. Some types of networks, however, cannot conduct such a high voltage signal without damaging the network; for example, a semiconductor network can conduct audio signals but not high voltage ringing signals. It is desirable in such a case to use an audio tone alerting signal which is in the same frequency and voltage range as the voice signals transmitted through the network. As a result, a number of different tone ringers operating on a variety of different principles have been implemented.
A problem exists at the time when a new office using tone alerting signals is cut over to replace an old office using conventional high voltage ringing signals. The problem occurs because tone ringer circuits and conventional ringer circuits are not compatible and therefore it is necessary to change all of the subscriber subsets at the same time the office is cut over. Since changing all of the subscriber subsets at one time is impractical a number of other solutions have been conceived although none of them has proven totally satisfactory. Examples of alternate solutions are the installation of new subscriber lines and subsets responsive to tone alerting while leaving the old subscriber subsets so that each subscriber had two telephone subsets and two lines for a period of time or the use of circuits in the telephone office for generating conventional ringing signals responsive to tone alerting signals so that the old subscriber subsets may be utilized with tone alerting.
It is an object of this invention to provide a subscriber subset that will respond to both conventional ringing signals and to tone alerting signals. Such a subscriber subset may be installed in place of the old subscriber subsets prior to cutover of a new office using tone alerting signals and then the new office may be cut over at any time without concern for the type of ringing signal used. In addition, such a subscriber subset built according to the invention disclosed retains the economical electromechanical ringer along with certain desirable testing features even after cutover to a tone alerting system.